you should just give him large algae disks that he cant fit in mouth so then he trys to eat that while the others eat thats wut i did with my JD and Red Devil
My jd is the same way, I bought him about 2 months ago at like 1". He's like 4" now, everytime, I walk by the tank he's always swimming spastically, opening his mouth wanting to be fed. I feed him twice a day with hikari gold medium sized pellets.
I have 4 JD's that are about 5" long in a 75 gallon with 1 large placo. They all eat like crazy, anything that falls into the tank. I feed the jacks on one side of the tank, wherever the placo isn't and then I feed an algae wafer to the placo by sticking my hand in the water and this keeps the JD's away and gives the placo enough time to eat. I don't recommend that you stick your hand in the tank all the time. I just haven't gotten around to getting a pair of tongs.
I concur that they will eat less when adults. I have a 10" Blue Dempsey, and he is always happy to get his food once a day, but will only eat a few bites a meal. His favorite: spirulina sticks...lol go figure, he likes the greens
i have a 5 inch male jack dempsey and he is the greediest in my tank. he eats raw chicken raw ground beef feeder rosy red minnows and goldfish and he is always looking for more. i have 3 other smaller jack dempseys and they havent grown as large as him. cichlids eat 3 times a day